FIG. 12 is a diagram for explaining a normal printing operation performed by a thermal transfer printer. A thermal transfer printer capable of color image printing uses, for example, an ink ribbon 4 on which color ink regions of yellow Y, magenta M, and cyan C and an overcoat OP region are arranged in the same order in a repeated manner along its longitudinal direction, and prints (forms) an image I on a rolled paper 10 by sequentially transferring the inks of different colors, etc., onto the paper 10, while transporting the ink ribbon 4 in the direction of arrow A1. In the normal printing operation, after sequentially transferring the yellow Y, magenta M, and cyan C inks and the overcoat OP onto the paper 10, the thermal transfer printer transports the paper 10 in the direction of arrow A2 and cuts its leading edge; then, the printer further transports the paper 10 in the direction of arrow A2 and cuts the trailing edge of the image I, thus discharging the printed page out of the printer.
In such printers, the printable image size is limited by the size of each color ink region of the ink ribbon 4, but a printing technique is known in the art which achieves a print of a size larger than the size of each color ink region of the ink ribbon 4 by first printing one image and then the next image in succession without cutting the paper 10. Such printing is hereinafter referred to as “panoramic printing”.
FIGS. 13(A) to 13(D) are diagrams for explaining a prior art panoramic printing method. If a plurality of images are simply printed in succession without cutting the paper 10, a blank space I3 will remain between the first image I1 and the second image I2 on the paper 10, as shown in FIG. 13(A). If, in order to eliminate this blank space I3, the first image I1 and the second image I2 are printed by partially overlapping their edges, as shown in FIG. 13(B), the print density of the image overlapping region Io will become higher than the print density of the other regions, thus showing the overlapping region Io visibly. In FIGS. 13(B) and 13(C), x represents the position along the longitudinal direction of the paper 10 (the direction of arrow A2 in FIG. 12), and f(x) represents the print density at position x.
In view of the above, there is proposed, for example, in patent documents 1 and 2, a method for adjusting the print density in the overlapping region Io of the two images by gradually decreasing the print density of the first image I1 toward its trailing edge (the edge nearer to the second image) and gradually increasing the print density of the second image I2 from its leading edge (the edge nearer to the first image), as shown in FIG. 13(C). On the other hand, in patent document 3, there is proposed a method for making the image connecting edges less visible by offsetting the connecting edges of the two images I1 and I2 in the sub-scanning transfer direction for each of the Y, M, and C colors and correcting the grayscale data of the overlapping region based on a predetermined correction factor for each line extending in the sub-scanning transfer direction.